The present invention relates generally to gas separator systems and the like, and more particularly to an adsorbent fractionation system including a pneumatically operated four way switching valve for removing moisture from air streams.
Multi-chamber adsorbent air and gas fractionators are widely known for carrying out a process of separating gaseous mixtures. Some examples of this type of adsorbent fractionators are disclosed in U.S. Pat. Nos. 5,256,174, 4,468,239, 4,552,570,4,247,311 and 3,258,899. Multi-chamber adsorbent fractionators are commonly used for air drying and generally include two adsorbent beds which are adapted for periodic cycling between an adsorbing or drying cycle and a desorption or regeneration cycle. The adsorbent beds include a drying agent, such as desiccant beads or particles, for removing moisture from the air. The apparatus also includes an inlet line for receiving a pressurized air feed stream, an exhaust valve for exhausting desorbed gas, and flow control valves for directing the air flow between the inlet line and exhaust valve and the beds. In addition, the apparatus includes a control device for controlling the cycling time and switching the air flow in predetermined, periodic cycles to alternately place each of the desiccant beds in communication with the pressurized air flow from the inlet line and the exhaust valve.
In operation, the air feed stream to be dried is passed through the first bed which is maintained under the substantially relatively high pressure of the original air feed stream and the dried air is discharged at substantially the original air feed stream pressure. When the adsorptive capacity of the first bed is reached, the control device activates the flow control valves to switch he original air feed stream to the second bed and the air is cycled to the second bed while the first bed is simultaneously depressurized or placed on the regeneration or desorption cycle by opening one end of the exhaust valve to a region of relatively low pressure, such as atmospheric pressure. At the same time as pressure is reduced, a lower pressure purge air flow tapped from the dried air discharge is introduced as reflux into the first bed to pass over and through the desiccant material therein and purge the moisture from and regenerate the bed. After the first bed is regenerated and the adsorptive capacity of the second bed is reached, the control device activates the flow control valves to switch the original air feed stream to the first bed and the cycle begins again. Thus, the periodic switching of the flow passages connecting the desiccant beds to the inlet and exhaust lines causes a pressurized adsorption process and a reduced pressure desorption and regeneration process to be alternately carried out in each of the desiccant beds.
The cycling times for periodically switching the beds from the adsorption cycle to the regeneration cycle and back to the adsorption cycle may be fixed or variable, depending on the system use. The device for controlling cycling times for the periodic switching of the beds at a predetermined time may be a sequencer, a timer, a microprocessor, or the like. While the determination and control of the cycling time can be accomplished using several different control devices, the task of actually carrying out the interchange of flow between the beds and reversing the air flow from one bed to the other is typically handled by an array of flow control valves. The flow control valves typically include an inlet valve for each bed, an exhaust valve for each bed, a depressurization or dump valve and a repressurization valve. One disadvantage of this arrangement is that the plurality of separate valves increases the weight of the apparatus and the distances between the separate valves increase the likelihood of undesirable pressure drops within the apparatus. Another disadvantage is that failure in a single valve can result in the malfunction of the entire system. Further, if electrically operated valves are used, the frequent cycling in this type of apparatus will result in high energy costs and possible malfunction due to an electrical fault or power failure or low voltage. Thus, it is desirable to limit the number of valves while still providing a flow control valve system which effectively and reliably switches the beds between the adsorption and regeneration cycles.
Removal of moisture from the air feed stream depends upon several factors including the rate of flow of the stream, the rate of moisture adsorption and moisture content of the adsorbent, as well as the temperature and pressure of the air within the bed. While the bed in the drying cycle is maintained at a relatively high pressure for optimum adsorption, the purge or regeneration of the saturated desiccant bed is ordinarily carried out at a pressure lower than the pressure of the adsorption or drying cycle. In order to effectively regenerate the absorbent in the bed on the regeneration cycle, it is important to completely depressurize the bed. Lower pressure during the regeneration process can result in dryer regeneration which is more efficient because it dries air to a lower level to remove fluids and regeneration is more effective.
Each time a cycling occurs and there is a switch between the pressurized adsorption process and the reduced-pressure desorption regeneration, a bed is depressurized by venting through the exhaust valve to the atmosphere. While complete depressurization is important for optimum operation of the system, one problem with depressurizing is that if the air is released through the exhaust valve too quickly at a high fluidization velocity, it can result in a noisy blast. Further, the blast from the exhaust flow may result in the churning or vibration of the desiccant beads in the adsorbent bed being depressurized. Thus, the fluidization velocity must be maintained or the desiccant particles may be fluidized and destroy or reduce the adsorbent capabilities of the bed when it is switched to the drying cycle.
In prior art systems, a separate depressurization or dump valve is constructed to help limit exhaust flow exiting from the exhaust valve and reduce noise and sorbent bed churning and abrasion during depressurizing of the adsorbent bed. An example of this type of apparatus; including a dump valve can be seen in Seibert U.S. Pat. No. 4,247,311. Seibert '311 is directed to a dryer comprising a pair of desiccant tanks and including an inlet line for distributing an influent gas to inlet valves which control the flow of influent gas to the tanks. The apparatus also includes a pair of exhaust valves connected to the tanks through which purge flow is vented to the atmosphere. A feature of Seibert '311 is a dump or exhaust flow valve that regulates or limits exhaust flow from a sorbent bed that is vented through the exhaust valves. The dump valve comprises a coil spring valve exposed on one side to the gas pressure in one of the two tanks through the exhaust valves and to atmospheric pressure on the other side. The coil spring valve includes a critical orifice for bleeding gas past the valve when the valve is in the closed position for regulating or limiting exhaust flow through the dump valve. When the exhaust valves are open at one end to atmospheric pressure to reduce the pressure from the pressurized adsorption process to place the bed on the regeneration or desorption cycle, the coil spring under the resulting pressure differential thereacross is compressed to a closed position. Although the coil spring is in the closed position, limited exhaust flow may proceed through the orifice and the pressure differential therein gradually diminishes as the exhaust flow is vented. As the pressure differential diminishes below the pressure at which damage to the adsorbent bed can result, the spring gradually opens to permit flow through the coils.
While this type of coil spring valve works to regulate exhaust flow and reduce noise and sorbent bed churning, one disadvantage is that the coil spring is repeatedly subjected to tremendous pressures and the spring is prone to breaking. When the exhaust valves are opened to atmospheric pressure, the switch between the pressurized-adsorption bed and the reduced-pressure regeneration bed cause a tremendous force to slam on the spring as the exhaust flow rushes out from the exhaust valves. This occurs during each cycle and typically results in the spring breaking after a period of time. If the spring breaks, the coil spring valve does not close and the exhaust flow therethrough is not regulated or limited. Thus, the blast from the exhaust flow vented to the atmosphere is not controlled and may result in the churning or vibration of the desiccant beads in the adsorbent bed being depressurized and may destroy or reduce the adsorbent capabilities of the bed. If the adsorbent capabilities of the bed are reduced, the air feed stream is not effectively dried and the entire system is affected. Thus, a failure in the coil spring valve can result in the malfunction of the entire system and a reliable valve for controlling exhaust flow through the exhaust valves that is not prone to breaking is needed.
In addition, each time a cycling occurs and there is a switch between the reduced pressure desorption regeneration cycle and the adsorption cycle, a regenerated bed is repressurized by switching the air feed stream to the regenerated bed. One problem with repressurization of a bed is that if the air feed stream is introduced into the bed too quickly at a high fluid velocity, it can result in the churning or vibration of the desiccant beads in the adsorbent bed being repressurized. Thus, the fluidization velocity must be maintained or the desiccant particles may be fluidized and destroy or reduce the adsorbent capabilities of the bed. The necessary fluid velocity and maximum rate of air flow which can be introduced into the bed without disturbing the desiccant within the adsorbent bed is calculated using well known equations and methods. While the rate of the air feed stream can be controlled at its source, the switching of the beds from the regeneration cycle to the adsorption cycle causes a blast of pressurized air to enter the bed being repressurized, so that the air feed stream is introduced into the bed at a rate that is too high for maintaining the necessary fluid velocity therethrough. Prior art systems may include a separate repressurization valve that is constructed to reduce the rate of air flow to the bed being repressurized for a controlled repressurization time. However, one problem with these systems is that the separate valve adds weight and complexity to the system and the distances between the separate valves may increase the likelihood of undesirable pressure drops within the system. Another problem is that the repressurization valve is repeatedly subjected to the blast of air pressure upon the switching of the beds and is therefore prone to breaking. If the valve breaks, the repressurization time is uncontrolled and the pressurized air enters the bed at an undesirable high velocity rate which may destroy the desiccant beads of the adsorbent bed. Thus, the failure of the repressurization valve can result in the malfunction of the entire system.
In accordance with the present invention, a switching valve for a multi-chamber adsorbent air and gas fractionation system is provided which simplifies the design and control of the switching valve by eliminating the need for separate repressurization and depressurization valves while providing a reliable valve and an optimum drying system with a controlled repressurization time and controlled depressurization process.